This invention relates to an electrical laminate. Reinforced unsaturated polyester resin laminates find important uses as electrical components such as printed circuit wiring boards. These laminates are conventionally produced by impregnating a fibrous substrate with unsaturated polyester resin (mixture of unsaturated polyester and a cross-linking vinyl monomer), laminating layers of resin-impregnated substrate and curing the resin under heat and pressure.
Cellulosic papers have many advantages as substrate for the production of electrical laminates but they tend to pick up moisture excessively thereby deteriorating in electrical properties of the entire structure. Their affinity to the resin is not necessarily satisfactory.
In an attempt to eliminate these and other defects, Japanese Patent Publication No. 13781/63 proposes to treat the cellulosic substrate with an aminoplast such as methylolmelamines, methylolguanamines and the like. Experiments have shown, however, that this treatment makes the resulting resin laminates to be excessively stiff and thus adversely affects on the impact strength and punchability of the laminates, though decrease in electrical insulation properties and solder dip resistance caused by moisture absorption may be effectively prevented.